[time-nuts] Lightning and grounds...

[Magnus Danielson]

Don Latham wrote:
> Hi Burt and all:
> Only a reminder: lightning is a pulse, and especially at the ground
> terminus can have frequency components in excess of tens of MHz. Think in
> terms of impedance rather than resistance.

I think the use of frequency components is a bit of missguiding when 
talking about a pulse. If concept as risetime is used instead, then a 
suitable comparision to sine frequency can be made.

A similar thing is found in ESD, where the finger has a risetime of 
about 0,7-1 ns... to say 8 kV, then the hand is discharged and the 
inductor (arm) finally conducts it will sent a second pulse as it 
discharges the body capacitance. The second pulse has the energy, the 
first pulse has the risetime. The hand pulse can act as a leader to the 
main bolt.

The return stroke of a ligthning has about 200 ns of risetime, can have 
current derivates peaking at 100 GA/s (induction!) and a charge transfer 
at 20 Coloumb. After the main dump a longer period having several 
hundred amps gets conducted in the ionised channel as the cloud 
redistribute charge in more conduction rather than inductive manner. 
Peak current can be some 20 kA.

The spark-gap (or com-gap) goes down to about 3 ns trigger time, unless 
you salt them with some ionating radiation in which they is about 1 ns 
of trigger time. It is prefered to have balanced ones, as one side can 
trigger the other... which is a good thing since if you use two single 
com-gaps and only one triggers you have just converted the jolt from 
common mode to diffrential mode... which defeats some of the 
protections. A problem with spark-gaps is that they may not be triggered 
since the rising edge is too slow. Thus, one wants to ensure that there 
is a low-inductance path through the spark-gap so that the bolt takes 
that path and also triggers it.

One has to recall that protection goes in stages in which over-voltage 
and over-currents gets smaller and smaller.

I haven't looked at any modern research on this, so I guess people have 
learned more. So the above may be outdated, but it should be enougth to 
give a ballpark hint on the properties.

The main difference between ESD and ligthning is the scale of things.

Trying to use a flask-brush to emit ions towards the clouds in hope to 
de-charge them in advance is meaningless if done on a single tower. 
Clouds typically move quickly on thunderstorms so you need to put alot 
of brushed up in a rather large circumference around whatever is 
sensitive stuff in order to have a main effect. I'm a sceptic here.
The leader goes down from the clouds and meet the streamer from the 
ground... the comes the return strike. A brush or ligthning rod just 
aids in selecting where the streamer from ground goes up and thus where 
the return strike hits.

I can't grasp how the brush-method significantly changes that basic 
behaviour.

Cheers,
Magnus